The present invention relates to dielectric resonator antennas (DRAs) composed of several adjacent segments, which may be excited simultaneously to provide steerable receive and transmit beams and very low backlobes.
Since the first systematic study of dielectric resonator antennas (DRAs) in 1983 [LONG, S. A., McALLISTER, M. W., and SHEN, L. C.: xe2x80x9cThe Resonant Cylindrical Dielectric Cavity Antennaxe2x80x9d, IEEE Transactions on Antennas and Propagation, AP-31, 1983, pp 406-412], interest has grown in their radiation patterns because of their high radiation efficiency, good match to most commonly used transmission lines and small physical size [MONGIA, R. K. and BHARTIA, P.: xe2x80x9cDielectric Resonator Antennasxe2x80x94A Review and General Design Relations for Resonant Frequency and Bandwidthxe2x80x9d, International Journal of Microwave and Millimetre-Wave Computer-Aided Engineering, 1994, 4, (3), pp 230-247].
The majority of configurations reported to date have used a slab of dielectric material mounted on a ground plane excited by either an aperture feed in the ground plane [ITTIPIBOON, A., MONGIA, R. K., ANTAR, Y. M. M., BHARTIA, P. and CUHACI, M: xe2x80x9cAperture Fed Rectangular and Triangular Dielectric Resonators for use as Magnetic Dipole Antennasxe2x80x9d, Electronics Letters, 1993, 29, (23), pp 2001-2002] or by a probe inserted into the dielectric material [McALLISTER, M. W., LONG, S. A. and CONWAY G. L.: xe2x80x9cRectangular Dielectric Resonator Antennaxe2x80x9d, Electronics Letters, 1983, 19, (6), pp 218-219]. Direct excitation by transmission lines has also been reported by some authors [KRANENBURG, R. A. and LONG, S. A.: xe2x80x9cMicrostrip Transmission Line Excitation of Dielectric Resonator Antennasxe2x80x9d, Electronics Letters, 1994, 24, (18), pp 1156-1157].
Further analysis of steerable-beam DRAs is to be found in the present applicant""s co-pending U.S. patent application Ser. No. 09/431,548, from which the present application claims priority and the disclosure of which is incorporated into the present application by reference.
Two of the most commonly described geometries are cylindrical and rectangular dielectric slabs. Several publications describe how these may be bisected through an image plane by a conducting sheet [TAM, M. T. K. and MURCH, R. D.: xe2x80x9cHalf volume dielectric resonator antenna designsxe2x80x9d, Electron. Lett., 1997, 33, (23), pp. 1914-1916; MONGIA, R. K.: xe2x80x98Half-split dielectric resonator placed on metallic plane for antenna applicationsxe2x80x99. Electron. Lett., 1989, 25, (7), pp 462-464]. To the applicant""s knowledge, only one publication describes antennas made from segments smaller than a half volume [TAM, M. T. K. and Murch, R. D.: xe2x80x9cCompact Circular Sector and Annular Sector Dielectric Resonator Antennasxe2x80x9d, IEEE Transactions on Antennas and Propagation, AP-47, 1999, pp 837-842].
According to a first aspect of the present invention, there is provided a compound dielectric resonator antenna comprising a plurality of individual dielectric resonator antennas, each including a grounded substrate, a dielectric resonator element having side faces and associated with the rounded substrate, and a feeding mechanism for transferring energy into and from the dielectric resonator element, characterised in that the dielectric resonator elements are arranged such that at least one side face of each dielectric resonator element is adjacent to at least one side face of a neighbouring dielectric resonator element and in that the antenna further includes electronic circuitry provided to activate the dielectric resonator elements individually or in combination so as to produce at least one incrementally or continuously steerable beam, which may be steered through a predetermined angle.
According to a second aspect of the present invention, there is provided a compound dielectric resonator antenna comprising a plurality of individual dielectric resonator antennas, each including a dielectric resonator element having side faces, and a feeding mechanism for transferring energy into and from the dielectric resonator element by way of at least one dipole feed. characterised in that the dielectric resonator elements are arranged such that at least one side face of each dielectric resonator element is adjacent to at least one side face of a neighbouring dielectric resonator element and in that the antenna further includes electronic circuitry provided to activate the dielectric resonator elements individually or in combination so as to produce at least one incrementally or continuously steerable beam, which may be steered through a predetermined angle.
It is preferred that the adjacent side faces are substantially contiguous. in that they contact each other. Alternatively, small gaps may be present between the adjacent side faces, these gaps being filled with air or another dielectric material.
Advantageously, the adjacent side faces of at least one pair of neighbouring dielectric resonator elements are separated by an electrically conductive wall which contacts both adjacent side faces. Preferably, all adjacent side walls are separated by an electrically conductive wall.
The dielectric resonator elements may be disposed directly on, next to or under the grounded substrate, or a small gap may be provided between the elements and the grounded substrate. The gap may comprise an air gap, or may be filled with another dielectric material of solid, liquid or gaseous phase.
The present invention seeks to provide an antenna having several elements, each of which is a segmented DRA. These elements may be excited simultaneously in order to provide steerable receive and transmit beams, radio direction finding capabilities, intelligent (or xe2x80x98smartxe2x80x99) antenna capabilities, low radiation backlobes and narrower radiation main lobes. The present invention also seeks to provide a significant further reduction in the backlobes by using extensions to the conducting walls that define the sides or edges of the DRA elements. Low backlobes are of particular importance to the application of these antennas to mobile telephones. Furthermore, an original geometry for the elements is proposed.
In some embodiments, a 90 degree sector of a cylindrical or annular DRA is resonated in its fundamental HEM21xcex4 mode, but there are several other resonant modes that may be used with this and with other geometries. An example of another combination is a 60 degree sector and its associated fundamental HEM31xcex4 mode.
The preferred HEM11xcex4, HEM21xcex4 and HEM31xcex4 modes are hybrid electromagnetic resonance modes, radiating like a horizontal magnetic dipole, which give rise to a vertically polarised radiation pattern with a cosine or figure-of-eight shaped pattern.
It has been noted by the present applicants that the results described in the above reference apply equally to DRAs operating at any of a wide range of frequencies, for example from 1 MHz to 100,000 MHz and even higher for optical DRAs. The higher the frequency in question, the smaller the size of the DRA, but the general beam patterns achieved by the probe/aperture and segment combination geometries described hereinafter remain generally the same throughout any given frequency range. Operation at frequencies substantially below 1 MHz is also possible, using dielectric materials with a high dielectric constant.
Advantageously, the antenna and antenna system of the present invention are adapted to produce at least one incrementally or continuously steerable beam, which may be steered through a complete 360 degree circle.
Advantageously, there is additionally or alternatively provided electronic circuitry to combine the feeds to form sum and difference patterns to permit radio direction finding capability of up to 360 degrees.
The electronic circuitry may additionally or alternatively be adapted to combine the feeds to form amplitude and/or phase comparison radio direction finding capability of up to 360 degrees.
In a first preferred embodiment, radio direction finding and beamforming capability is a complete 360 degree circle, with the individual DRA elements being arranged in a generally circular configuration about a longitudinal axis with each element being flanked by two neighbouring elements. It is to be understood that the elements need not be shaped so as to have cross-sections which form sectors of a circle. Instead, the elements may have generally triangular or trapezoidal cross-sections, the main consideration being that the elements are shaped so as to fit together about a longitudinal axis with each element being flanked by two neighbouring elements.
In a second preferred embodiment, radio direction finding and beamforming capability is less than a complete circle using an array of elements disposed about a longitudinal axis which themselves amount to less than a circle, with all except the first and last elements of the array being flanked by two neighbouring elements.
In both first and second embodiments, it is preferred that all the elements making up the DRA have the same cross-section. This means that each element will behave in a similar manner to the others when excited, notwithstanding directional effects due to the relative orientations of the elements.
One method of electronically steering an antenna pattern is to have a number of existing beams and to switch between them. An alternative method is to combine them so as to achieve the desired beam direction. With DRAs, the antenna patterns are essentially cosine shaped and adding together two cosines slightly displaced in angle gives a third cosine pattern half way between the two. In this way, beam steering and direction finding may be achieved by combining fixed antenna patterns.
The advantage of direction finding is that the direction of a base station can be found (by a mobile phone for example) and the advantage of beam steering is that a beam can then be formed in the direction of the base station. These advantages combine to improve the transfer of power between a mobile phone and base station, thereby increasing communication quality and conserving battery life, and yet, simultaneously, reducing the transfer of power into the body of the person using the phone. An important finding by the present applicant is that a single element driven alone does not generally have a backlobe as small as, say, two elements driven simultaneously. The simultaneous use of at least two elements can confer a significant advantage in this respect.
An advantage of the geometry of the second preferred embodiment above and similar geometries, wherein the elements are not arranged in a complete circle, is that the backlobe generated by the antenna which irradiates nearby objects (such as the human head when using mobile phones) can, with some geometrical arrangements, be kept very low thereby much reducing the irradiation and resulting in improved safety.
A further advantage of the geometry of the second preferred embodiment and similar geometries, is that the main lobe generated by the antenna can be narrower when two elements are excited together than for either element separately.
A further reduction in the backlobe of a segmented DRA can be obtained by providing extensions to the conducting walls that define the edges of each element. Such devices can be simply planar extensions of the conducting walls, but they may also be curled, or deformed in other ways, so as to impede the electromagnetic wave trying to creep round the edge of the wall and so create (or contribute to) the backlobe of the antenna. This has been demonstrated by the present applicant using a half-cylinder DRA resonating at 58 MHz.
In a further embodiment of the present invention, there may additionally be provided at least one internal or external monopole antenna or any other antenna possessing a circularly symmetric pattern about a longitudinal axis, which is combined with at least one of the dielectric resonator antenna elements so as to cancel out backlobe fields or to resolve any front-to-back ambiguity which may occur with a dielectric resonator antenna having a cosine or figure-of-eight radiation pattern. The monopole or other circularly symmetrical antenna may be centrally disposed within the dielectric resonator element or may be mounted thereupon therebelow and is activatable by the electronic circuitry. In embodiments including an annular resonator with a hollow centre, the monopole or other circularly symmetrical antenna may be located within the hollow centre. A xe2x80x9cvirtualxe2x80x9d monopole or other circularly symmetrical antenna may also be formed by an electrical or algorithmic combination of any of the actual feeds, preferably a symmetrical set of feeds.
With all the segment geometries described above, the feeds may take the form of conductive probes which are contained within or placed against the dielectric resonator elements, or a combination thereof, or may comprise aperture feeds provided in the grounded substrate. Aperture feeds are discontinuities (generally rectangular in shape) in the grounded substrate underneath the dielectric material and are generally excited by passing a microstrip transmission line beneath them. The microstrip transmission line is usually printed on the underside of the substrate. Where the feeds take the form of probes, these may be generally elongate in form. Examples of useful probes include thin cylindrical wires which are generally parallel to a longitudinal axis of the dielectric resonator. Other probe shapes that might be used (and have been tested) include fat cylinders, non-circular cross sections, thin generally vertical plates and even thin generally vertical wires with conducting xe2x80x9chatsxe2x80x9d on top (like toadstools). Probes may also comprise metallised strips placed within or against the dielectric, or a combination thereof. In general, any conducting element within or against the dielectric resonator, or a combination thereof, will excite resonance if positioned, sized and fed correctly. The different probe shapes give rise to different bandwidths of resonance and may be disposed in various positions and orientations (at different distances along a radius from the centre and at different angles from the centre, as viewed from above) within or against the dielectric resonator or a combination thereof, so as to suit particular circumstances. Furthermore, there may be provided probes within or against the dielectric resonator, or a combination thereof, which are not connected to the electronic circuitry but instead take a passive role in influencing the transmit/receive characteristics of the dynamic resonator antenna, for example, by way of induction.
Generally, where the feed comprises a monopole feed, then the appropriate dielectric resonator element must be associated with a rounded substrate, for example by being disposed thereupon or separated therefrom by a small air gap or a layer of another dielectric material. Alternatively, where the feed comprises a dipole feed, then no grounded substrate is required. Embodiments of the present invention may use monopole feeds to dielectric elements associated with a grounded substrate, and/or dipole feeds to dielectric elements not having an associated grounded substrate. Both types of feed may be used in the same compound antenna.
The dielectric resonator elements may be segments of a cylinder, having substantially radial conducting walls advantageously disposed generally parallel to the longitudinal axis.
Alternatively, the dielectric resonator elements may be of a generally trapezoidal cross-section, having conducting walls advantageously disposed generally parallel to the longitudinal axis.
The array of elements may be arranged so as to be with or without a hollow centre.
The dielectric resonator elements may have cross-sections other than segments of a circle or generally trapezoidal. What is important for achieving the greatest backlobe reductions is that the array of elements has full or at least partial circular symmetry about the longitudinal axis.
The dielectric resonator antenna of the present invention may be operated with a plurality of transmitters or receivers, the terms here being used to denote respectively a device acting as a source of electronic signals for transmission by way of the antenna or a device acting to receive and process electronic signals communicated to the antenna by way of electromagnetic radiation. The number of transmitters and/or receivers may or may not be equal to the number of elements being excited. For example, a separate transmitter and/or receiver may be connected to each element (i.e. one per element), or a single transmitter and/or receiver to a single element (i.e. a single transmitter and/or receiver is switched between elements). In a further example, a single transmitter and/or receiver may be (simultaneously) connected to a plurality of elements. By continuously varying the feed power between the elements, the beam and/or directional sensitivity of the antenna may be continuously steered. A single transmitter and/or receiver may alternatively be connected to several non-adjacent elements. In yet another example, a single transmitter and/or receiver may be connected to several adjacent or non-adjacent elements in order to produce an increase in the generated or detected radiation pattern, or to allow the antenna to radiate or receive in several directions simultaneously.
The dielectric resonator elements may be formed of any suitable dielectric material, or a combination of different dielectric materials, having an overall positive dielectric constant k. In preferred embodiments, k is at least 10 and may be at least 50 or even at least 100. k may even be very large, e.g. greater than 1000, although available dielectric materials tend to limit such use to low frequencies. The dielectric material may include materials in liquid, solid or gaseous states, or any intermediate state. The dielectric material could be of lower dielectric constant than a surrounding material in which it is embedded.
By seeking to provide a dielectric resonator antenna capable of generating multiple beams, which can be selected separately or formed simultaneously and combined in different ways at will, embodiments of the present invention may provide the following advantages:
i) By choosing to drive different elements of a multi-element DRA, the antenna can be made to transmit or receive in one of a number of preselected directions (in azimuth, for example). By sequentially switching round the elements, the beam pattern can be made to rotate incrementally in angle. Such beam-steering has obvious applications for radio communications, radar and navigation systems.
ii) By combining two or more beams together, i.e. exciting two or more elements simultaneously, beams can be formed in any arbitrary azimuth direction, thus giving more precise control over the beamforming process.
iii) By electronically continuously varying the power division/combination between two beams, the resultant combination beam direction can be steered continuously.
iv) On receive only, the direction of arrival of an incoming radio signal can be found by comparing the amplitude of the signal on two or more beams, or by carrying out monopulse processing of the signal received on two beams. xe2x80x9cMonopulse processingxe2x80x9d refers to the process of forming sum and difference patterns from two beams so as to determine the direction of arrival of a signal from a distant radio source.
v) In a typical two-way communication system (such as a mobile telephone system) signals are received (by a handset) from a point radio source (such as a base station) and transmitted back to that source. Embodiments of the present invention may be used to find the direction of the source using iii) or iv) above and may then form an optimal beam in that direction using ii). An antenna capable of performing this type of operation is said to have as a xe2x80x9csmartxe2x80x9d or xe2x80x9cintelligentxe2x80x9d capabilities. The advantages of the improved antenna gain offered by smart antennas is that the signal-to-noise ratio is improved, communications quality is improved, less transmitter power may be used (which can, for example, help to reduce irradiation of any nearby human body) and battery life is conserved.
vi) Beamsteering and smart antenna technology may also be used to steer a sharp null in a particular direction to avoid transmitting there or to avoid receiving interfering signals from that direction.